Photo-acoustic gas sensors operate by heating a gas using a laser tuned to a gas absorption line corresponding to a gas to be detected, such as hydrogen sulfide. If the gas is present even at part per million levels optical absorption occurs, which causes local heating and results in thermal expansion (pressure waves). The pressure waves are an acoustic signature that may be detected by a microphone or other acoustic pressure responsive sensor.
An alternative mechanism for measuring the presence of a specific gas includes a direct extinction method that measures light intensity before and after a gas absorption cell. Conclusions can be drawn regarding the concentration of the light-absorbing gas from the difference of the two measurements. This method permits analysis in flowing gas. However, it is only suitable for determination of relatively high gas concentrations, unless very considerable cell lengths or multiple reflection cells can be accepted. However, in the latter case it is necessary for the incident light beam to be narrowly focused and the mirror system must be very accurately adjusted. Multiple reflection cells used in such a mirror system are generally very complicated and costly. Environmental factors can adversely affect the performance of such mirror systems.